Magnetic particle inspection (MPI) is a non-destructive testing (NDT) method for detecting surface and subsurface discontinuities in ferrous materials. The process puts a magnetic field into the to be examined object. The presence of a surface or subsurface discontinuity in the material allows magnetic flux to leak.
Ferrous iron particles are applied to the part. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken if any.